In general, polyimide has been applied to various fields including insulating substrates for forming circuits and devices due to excellent mechanical properties and heat properties. However, since a charge transfer complex is formed between aromatic rings during polymerization, the polyimide is colored with brown or yellow color, and thus, a transmittance in a visible light region is low, which has difficulty in being applied to display materials.
As a method of preparing the polyimide to be colorless and transparent, a method of suppressing formation of the charge transfer complex in a molecule by using alicyclic diamine or aliphatic diamine as a diamine component has been known. Japanese Patent Laid-Open Publication No. 2002-161136 (Patent Document 1) discloses polyimide obtained by imidization of a polyimide precursor formed with an aromatic acid dianhydride such as pyromellitic dianhydride, etc., and trans-1,4-diaminocyclohexane. Even though the polyimide exhibits high transparency, there is a problem in that mechanical properties are deteriorated.
Various methods for converting the yellow color of polyimide into colorless transparency have been attempted. However, there is difficulty in approaching the conversion due to problems in a preparation process, such as a rapid increase in viscosity during polymerization or difficulty in refining, etc. For example, a method of using a functional monomer, a method of using a solvent containing an inorganic salt, or the like, has been attempted, but it is still insufficient to simultaneously implement optical properties and mechanical properties, and there is a problem in that deterioration in physical properties inherent to polyimide is inevitable.
Meanwhile, a range of application of the display materials has been diversified according to development of various electronic devices. Research and development for replacing a cover glass for a display with a polymer material have been actively conducted. Thus, it is urgently required to develop a technique for improving optical characteristics while simultaneously implementing excellent mechanical properties inherent to polyimide in various display material fields including a material for replacing the cover glass.